Lithographic apparatus and device manufacturing method

ABSTRACT

A lithographic apparatus and device manufacturing method makes use of a high refractive index liquid confined in a reservoir  13  at least partly filling the imaging field between the final element of the projection lens and the substrate. Bubbles forming in the liquid from dissolved atmospheric gases or from out-gassing from apparatus elements exposed to the liquid are detected and removed so that they do not interfere with exposure and lead to printing defects on the substrate. Detection can be carried out by measuring the frequency dependence of ultrasonic attenuation in the liquid and bubble removal can be implemented by degassing and pressurizing the liquid, isolating the liquid from the atmosphere, using liquids of low surface tension, providing a continuous flow of liquid through the imaging field, and phase shifting ultrasonic standing-wave node patterns.

FIELD OF THE INVENTION

The present invention relates to a lithographic projection apparatus.

In particular, the present invention relates to a lithographicprojection apparatus comprising a radiation system for supplying aprojection beam of radiation, a support structure for supportingpatterning means, the patterning means serving to pattern the projectionbeam according to a desired pattern, a substrate table for holding asubstrate, a projection system for projecting the patterned beam onto atarget portion of the substrate, and liquid supply system for at leastpartly filling a space between the final element of said projectionsystem and said substrate with liquid.

SUMMARY

The term “patterning means” as here employed should be broadlyinterpreted as referring to means that can be used to endow an incomingradiation beam with a patterned cross-section, corresponding to apattern that is to be created in a target portion of the substrate; theterm “light valve” can also be used in this context. Generally, the saidpattern will correspond to a particular functional layer in a devicebeing created in the target portion, such as an integrated circuit orother device (see below). Examples of such patterning means include:

A mask. The concept of a mask is well known in lithography, and itincludes mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. Placementof such a mask in the radiation beam causes selective transmission (inthe case of a transmissive mask) or reflection (in the case of areflective mask) of the radiation impinging on the mask, according tothe pattern on the mask. In the case of a mask, the support structurewill generally be a mask table, which ensures that the mask can be heldat a desired position in the incoming radiation beam, and that it can bemoved relative to the beam if so desired.

A programmable mirror array. One example of such a device is amatrix-addressable surface having a viscoelastic control layer and areflective surface. The basic principle behind such an apparatus is that(for example) addressed areas of the reflective surface reflect incidentlight as diffracted light, whereas unaddressed areas reflect incidentlight as undiffracted light. Using an appropriate filter, the saidundiffracted light can be filtered out of the reflected beam, leavingonly the diffracted light behind; in this manner, the beam becomespatterned according to the addressing pattern of the matrix-addressablesurface. An alternative embodiment of a programmable mirror arrayemploys a matrix arrangement of tiny mirrors, each of which can beindividually tilted about an axis by applying a suitable localizedelectric field, or by employing piezoelectric actuation means. Onceagain, the mirrors are matrix-addressable, such that addressed mirrorswill reflect an incoming radiation beam in a different direction tounaddressed mirrors; in this manner, the reflected beam is patternedaccording to the addressing pattern of the matrix-addressable mirrors.The required matrix addressing can be performed using suitableelectronic means. In both of the situations described hereabove, thepatterning means can comprise one or more programmable mirror arrays.More information on mirror arrays as here referred to can be gleaned,for example, from U.S. Pat. No. 5,296,891 and U.S. Pat. No. 5,523,193,and PCT patent applications WO 98/38597 and WO 98/33096, which areincorporated herein by reference. In the case of a programmable mirrorarray, the said support structure may be embodied as a frame or table,for example, which may be fixed or movable as required.

A programmable LCD array. An example of such a construction is given inU.S. Pat. No. 5,229,872, which is incorporated herein by reference. Asabove, the support structure in this case may be embodied as a frame ortable, for example, which may be fixed or movable as required.

For purposes of simplicity, the rest of this text may, at certainlocations, specifically direct itself to examples involving a mask andmask table; however, the general principles discussed in such instancesshould be seen in the broader context of the patterning means ashereabove set forth.

Lithographic projection apparatus can be used, for example, in themanufacture of integrated circuits (ICs). In such a case, the patterningmeans may generate a circuit pattern corresponding to an individuallayer of the IC, and this pattern can be imaged onto a target portion(e.g. comprising one or more dies) on a substrate (silicon wafer) thathas been coated with a layer of radiation-sensitive material (resist).In general, a single wafer will contain a whole network of adjacenttarget portions that are successively irradiated via the projectionsystem, one at a time. In current apparatus, employing patterning by amask on a mask table, a distinction can be made between two differenttypes of machine. In one type of lithographic projection apparatus, eachtarget portion is irradiated by exposing the entire mask pattern ontothe target portion in one go; such an apparatus is commonly referred toas a wafer stepper. In an alternative apparatus—commonly referred to asa step-and-scan apparatus—each target portion is irradiated byprogressively scanning the mask pattern under the projection beam in agiven reference direction (the “scanning” direction) while synchronouslyscanning the substrate table parallel or anti-parallel to thisdirection; since, in general, the projection system will have amagnification factor M (generally <1), the speed V at which thesubstrate table is scanned will be a factor M times that at which themask table is scanned. More information with regard to lithographicdevices as here described can be gleaned, for example, from U.S. Pat.No. 6,046,792, incorporated herein by reference.

In a manufacturing process using a lithographic projection apparatus, apattern (e.g. in a mask) is imaged onto a substrate that is at leastpartially covered by a layer of radiation-sensitive material (resist).Prior to this imaging step, the substrate may undergo variousprocedures, such as priming, resist coating and a soft bake. Afterexposure, the substrate may be subjected to other procedures, such as apost-exposure bake (PEB), development, a hard bake andmeasurement/inspection of the imaged features. This array of proceduresis used as a basis to pattern an individual layer of a device, e.g. anIC. Such a patterned layer may then undergo various processes such asetching, ion-implantation (doping), metallization, oxidation,chemo-mechanical polishing, etc., all intended to finish off anindividual layer. If several layers are required, then the wholeprocedure, or a variant thereof, will have to be repeated for each newlayer. Eventually, an array of devices will be present on the substrate(wafer). These devices are then separated from one another by atechnique such as dicing or sawing, whence the individual devices can bemounted on a carrier, connected to pins, etc. Further informationregarding such processes can be obtained, for example, from the book“Microchip Fabrication: A Practical Guide to Semiconductor Processing”,Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN0-07-067250-4, incorporated herein by reference.

For the sake of simplicity, the projection system may hereinafter bereferred to as the “lens”; however, this term should be broadlyinterpreted as encompassing various types of projection system,including refractive optics, reflective optics, and catadioptricsystems, for example. The radiation system may also include componentsoperating according to any of these design types for directing, shapingor controlling the projection beam of radiation, and such components mayalso be referred to below, collectively or singularly, as a “lens”.Further, the lithographic apparatus may be of a type having two or moresubstrate tables (and/or two or more mask tables). In such “multiplestage” devices the additional tables may be used in parallel, orpreparatory steps may be carried out on one or more tables while one ormore other tables are being used for exposures. Dual stage lithographicapparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO98/40791, incorporated herein by reference. Where reference is made to“ultrasonic” or “ultrasound”, unless stated otherwise, this is to beinterpreted as relating to sound waves at any frequency greater than theupper limit of human perception: namely, greater than 20 kHz.

It has been proposed to immerse the substrate in a lithographicprojection apparatus in a liquid having a relatively high refractiveindex, e.g. water, so as to fill the space between the final element ofthe projection lens and the substrate. The point of this is to enableimaging of smaller features since the exposure radiation will have ashorter wavelength in the liquid. (The effect of the liquid may also beregarded as increasing the effective NA of the system.)

One of the solutions proposed is to submerse the substrate or substrateand substrate table in a bath of liquid (see for example U.S. Pat. No.4,509,852, hereby incorporated in its entirety by reference). In thissolution, there is a large body of liquid that must be acceleratedduring a scanning exposure. This may require additional or more powerfulmotors and may cause turbulence in the liquid leading to possibleundesirable and unpredictable effects.

Another of the solutions proposed is for a liquid supply system toprovide liquid in a localized area between the final element of theprojection system and the substrate (the substrate generally has alarger surface area than the final element of the projection system).One way that has been proposed to arrange for this is disclosed in WO99/49504, hereby incorporated in its entirety by reference. Asillustrated in FIGS. 2 and 3, liquid is supplied by at least one inletIN onto the substrate, preferably along the direction of movement of thesubstrate relative to the final element, and is removed by at least oneoutlet OUT after having passed under the projection system. That is, asthe substrate is scanned beneath the element in a −X direction, liquidis supplied at the +X side of the element and taken up at the −X side.FIG. 2 shows the arrangement schematically in which liquid is suppliedvia inlet IN and is taken up on the other side of the element by outletOUT which is connected to a low pressure source. In the illustration ofFIG. 2 the liquid is supplied along the direction of movement of thesubstrate relative to the final element, though this does not need to bethe case. Various orientations and numbers of inlets and outletspositioned around the final element are possible, one example isillustrated in FIG. 3 in which four sets of an inlet with an outlet oneither side are provided in a regular pattern around the final element.

Another solution that has been proposed is to provide the liquid supplysystem with a seal member which extends along at least a part of aboundary of the space between the final element of the projection systemand the substrate table. The seal member is substantially stationaryrelative to the projection system in the XY plane and a seal is formedbetween the seal member and the surface of the substrate. Preferably theseal is a contactless seal such as a gas seal (see, for example,European patent application 03252955.4 hereby incorporated in itsentirety by reference).

Unexpected disadvantages emerge from this new technology when comparedwith systems that do not have liquid in the exposure radiation path. Inparticular, despite the improved imaging resolution, the liquid tends todegrade the image quality in other respects.

It is an object of the present invention to improve the imagingperformance of an apparatus having a liquid filling a space between thefinal element of the projection system and the substrate.

This and other objects are achieved according to the invention in alithographic apparatus as specified in the opening paragraph,characterized in that the liquid supply system comprises bubblereduction means.

It has been realized that an important source of image degradation isthe scattering of imaging radiation from bubbles in the liquid. Byreducing the size and concentration of these bubbles it is possible toreduce this scattering and the associated distortion of the imagereaching the substrate, thereby reducing the frequency and magnitude ofdefects in the printed pattern on the substrate. Bubbles typically formwhen dissolved gases from the atmosphere come out of solution due to adisturbance of some kind, or from out-gassing elements of thelithographic apparatus, such as a photosensitive layer on the substrate.Bubbles thus formed may vary greatly in number density and sizedistribution depending on the liquid, gases and disturbances involved.Very fine bubbles tend to cause particular problems as they are bothdifficult to detect and hard to remove using standard methods and yetstill influence the image formed on the substrate. For use in thecontext of a typical lithographic apparatus, for example, bubblescontinue to degrade performance down to around 10 nm in diameter.

The bubble reduction means may comprise bubble detection means. It ispreferred that the bubble detection means comprise one or moreultrasonic transducers. These transducers may emit ultrasonic waves andreceive ultrasonic waves that are influenced by the presence of bubblesin the liquid within which they propagate. The information yielded bythe ultrasonic transducers may include information about thedistribution of bubble sizes as well as their number density.

The ultrasonic transducers may also measure the ultrasonic attenuationas a function of frequency. The advantage of this approach is that it ispossible to detect bubbles with dimensions very much smaller than thewavelength of the ultrasonic waves. Using only the amplitude of thesignal would restrict this measurement method to bubbles of the samesize or greater than the wavelength of the ultrasonic waves.

A further feature is that the bubble reduction means comprises a bubbleremoval means.

The bubble removal means may comprise a degassing device, the degassingdevice comprising an isolation chamber, wherein a space above liquid inthe isolation chamber is maintained at a pressure below atmosphericpressure encouraging previously dissolved gases to come out of solutionand be pumped away. This degassing process dramatically reduces theoccurrence of bubbles due to dissolved atmospheric gases coming out ofsolution. Following the degassing process, the liquid is preferably keptas isolated as possible from the normal atmosphere.

A further feature is that the bubble removal means provide a continuousflow of liquid over the final element of the projection system and thesubstrate in order to transport bubbles out of the imaging field. Thisstep is particularly effective for removing gases originating fromout-gassing elements of the lithographic apparatus.

Additionally, the bubble reduction means may pressurize the liquid aboveatmospheric pressure to minimize the size of bubbles and encouragebubble-forming gases to dissolve into the liquid.

The composition of the liquid may also be chosen to have a lower surfacetension than water. This reduces the tendency of bubbles to stick to thesubstrate where they may be particularly damaging to the image and wherethey tend to be resistant to removal measures. The tendency of bubblesto stick to the substrate and other components may be reduced bycontrolling the surface finish in contact with the immersion liquid. Inparticular, the surface finish may be polished or arranged to have aminimal surface roughness, preferably with a characteristic length scaleof less than 0.5 μm.

The bubble reduction means may treat the liquid before it is introducedinto the space between the final element of the projection system andthe substrate. An advantage of this approach is improved spaceconsiderations and liberty of design. These factors make it easier totreat liquid in bulk for use in a plurality of lithographic apparatusesor for use in a circulatory system or where the liquid is to be replacedon a frequent basis. After treatment, the liquid may be protected fromatmospheric gases by being kept under vacuum or by being exposed only toa gas, such as nitrogen, argon or helium, which does not easily dissolveinto the liquid.

The ultrasonic transducers of the bubble detection means may be arrangedin a pulse-echo arrangement wherein the same transducer emits waves and,after reflection from a boundary, receives waves attenuated bypropagation through the liquid. An advantage of this arrangement is thatfewer transducers are required and it is easier to arrange a relativelylong signal path through the liquid.

Alternatively, the bubble detection means may comprise two spatiallyseparated ultrasonic transducers, the first arranged to transmit, andthe second to receive waves. An advantage of this arrangement is thatthe signal received at the receiving transducer may be easier tointerpret and may suffer less from anomalous signal loss caused, forexample, by non-specular reflection from the boundary.

Optionally, the bubble removal means may include two spatially separatedultrasonic transducers, arranged to produce ultrasonic standing-wavepatterns within the liquid that trap bubbles within the nodal regions.The bubble removal means is arranged to displace said bubbles throughthe use of phase adjusting means linked with the transducers, the phaseadjusting means causing spatial shift of the nodal regions and ofbubbles trapped within them. This process may be used to transportbubbles completely to one side of a liquid reservoir where they may beisolated and removed from the system.

The ultrasonic transducers may preferably operate at megasonicfrequencies (in the region of 1 MHz). Megasonic waves avoid some of thedisadvantages of conventional (lower frequency) ultrasonic waves such ascavitation and bubble collision with solid surfaces, which results insmall particles being dislodged and contaminating the liquid.

The bubble removal means may comprise an electric field generator forapplying an electric field to the liquid, which electric field beingcapable of dislodging bubbles attached to interfaces within the liquid.This feature may be particularly useful where an interface in questionis the substrate, as bubbles attached here are at the focus of thelithographic projection apparatus and may therefore distort the imagemore severely. The electric field lines are distorted in the vicinity ofthe bubble, which has a dielectric constant different from that of thesurrounding liquid. This embodiment works on the basis that when thebubble is close to, or in contact with, an interface, the electric fielddistribution may be such as to force the bubble away from the surfaceand into the bulk of the liquid. Once in the bulk of the liquid, thebubble has a less detrimental effect on image quality, and may also bemore easily removed. This method is applicable even where the surface towhich the bubble has attached is hydrophobic and reduces the need toapply special hydrophilic coatings to the substrate.

The bubble removal means may comprise a selective heater for selectivelycontrolling the temperature and therefore the size of bubbles accordingto their composition. By selecting to heat only the bubbles and not thesurrounding liquid, it is possible to minimize unnecessary variations inthe liquid temperature. Increasing the temperature of the bubbles causesthem to expand in size and therefore become easier to remove. Theselective heater may comprise a microwave source, operating atfrequencies that correspond to the resonant frequencies of the gasmolecules forming the bubbles (commonly nitrogen and oxygen). Given thetemperature sensitivity of the lithographic apparatus in the region ofthe substrate, this method allows more extensive heating of the gaswithin the bubbles than would be the case if the liquid and bubbles hadto be heated simultaneously. The result is a more energy and timeefficient method for removing bubbles from the liquid.

The bubble removal means may comprise a particle input device forintroducing particles into the liquid, and a particle removal device forremoving the particles from the liquid. This method operates on theprinciple that, where the particles are chosen so that it isenergetically or otherwise favorable, gas bubbles tend to attach to thesurface of particles present in the liquid. Cumulatively, the particlespresent a large surface area to the liquid, which increases the chancesof contact between particle and bubble. The surface in question maycomprise the outer surface, and, where the particles are porous, theinternal surface associated with the pores. Porous particles thereforeprovide a larger particle surface in contact with the liquid thannon-porous particles. This embodiment is particularly effective when theparticles are arranged to have a surface that repels the liquid (i.e. asurface that has a high interface energy with the liquid). In the caseof a liquid comprising water, such a surface may be described ashydrophobic. This arrangement favors attachment of the bubbles as theyact to reduce the particle surface area in contact with the liquid, thusminimizing the surface energy. There may also be an electrostaticattraction between the bubble and particle, or other surfacecharacteristics of the particle that favor attachment of bubbles.

Gas bubbles that become attached to the particles are removed from theliquid when the particles are removed from the liquid by the particleremoval device. The particle removal device may comprise a particlefilter. In general, the dimensions of the particle are chosen to makethem easy to remove, and this method provides an efficient means forremoving even very fine bubbles.

The bubble detection means may comprise a light source, a light detectorand a light comparator. The light source and the light detector may bearranged so that light emitted by the source propagates between thesource and the detector through a portion of the liquid, the comparatorbeing arranged to detect changes in the proportion of the emitted lightthat arrives at the detector after propagation through a portion of theliquid. The presence of bubbles in the liquid causes the light to bescattered. Depending on the arrangement of the source and the detector,this scattering may cause an increase or a decrease in the signaldetected at the detector and may be analyzed to provide informationabout the population of bubbles. An advantage of this arrangement isthat it can be operated continuously, even when the projection apparatusis in normal operation. When bubbles occur, they can be detected at anearly stage and exposure can be suspended until the liquid is clearagain. This feature therefore minimizes lost time, and also reduces thequantity of poorly exposed substrates that are produced.

According to a further aspect of the invention there is provided alithographic projection apparatus comprising:

-   -   a radiation system for providing a projection beam of radiation;    -   a support structure for supporting patterning means, the        patterning means serving to pattern the projection beam        according to a desired pattern;    -   a substrate table for holding a substrate;    -   a projection system for projecting the patterned beam onto a        target portion of the substrate;    -   a liquid supply system for at least partly filling a space        between the final element of said projection system and said        substrate with liquid; and    -   a detection system for detecting impurities in said liquid,        including a light source, a light detector and a light        comparator, said light source and said light detector being        arranged so that light emitted by said source propagates between        said source and said detector through a portion of said liquid,        said comparator being arranged to detect changes in the        proportion of said emitted light that arrives at said detector        after propagation through a portion of said liquid.

The detection system may be arranged to detect particles in the liquidbetween the final element of the projection system and the substrate.Particles may be introduced deliberately in order to control opticalproperties of the liquid and enhance the performance of the lithographicapparatus. This may be achieved, for example, by a fine suspension ofquartz particles. In this case, the detection system may be used toverify that the particles are present in the desired proportions.Alternatively, damaging particles may enter the system by accident, suchas those that break away from surfaces in contact with the immersionliquid. In this case, the detection system may be used to detect theseparticles and initiate an alarm procedure when the particleconcentration and/or size distribution exceeds predetermined thresholds.Early detection of problems (whether a lack of desired particles or anexcess of undesirable particles) allows corrective action to be takenpromptly and helps to minimize loss of time and materials associatedwith substandard imaging.

According to a further aspect of the invention there is provided adevice manufacturing method comprising the steps of:

-   -   providing a substrate that is at least partially covered by a        layer of radiation-sensitive material;    -   providing a projection beam of radiation using a radiation        system;    -   using patterning means to endow the projection beam with a        pattern in its cross-section;    -   projecting the patterned beam of radiation onto a target portion        of the layer of radiation-sensitive material;    -   providing a liquid supply system for filling the space between        the final element of the projection system and said substrate        with liquid; and    -   reducing bubbles in said liquid supply system.

According to a still further aspect of the invention there is provided alithographic projection apparatus comprising:

-   -   a radiation system for providing a projection beam of radiation;    -   a support structure for supporting patterning means, the        patterning means serving to pattern the projection beam        according to a desired pattern;    -   a substrate table for holding a substrate;    -   a projection system for projecting the patterned beam onto a        target portion of the substrate;    -   a liquid supply system for at least partly filling a space        between the final element of said projection system and said        substrate with liquid; and    -   a liquid quality monitor capable of switching the operational        state of the projection apparatus between an active state and a        suspended state, said active state being selected when the        liquid quality is determined to be above a predefined threshold        and said suspended state being selected when the liquid quality        is determined to be below a predefined threshold.

This feature allows early detection of faults, and avoids unnecessaryloss of time and material due to faulty exposure of the substrates. Thepredefined thresholds may be based on parameters such as limits on thesize and/or number distribution of bubbles as detected by the bubbledetection means. Alternatively, the predefined thresholds may relate tolimits on the size and/or number distribution of other particles in theliquid.

Although specific reference may be made in this text to the use of theapparatus according to the invention in the manufacture of ICs, itshould be explicitly understood that such an apparatus has many otherpossible applications. For example, it may be employed in themanufacture of integrated optical systems, guidance and detectionpatterns for magnetic domain memories, liquid-crystal display panels,thin-film magnetic heads, etc. The skilled artisan will appreciate that,in the context of such alternative applications, any use of the terms“reticle”, “wafer” or “die” in this text should be considered as beingreplaced by the more general terms “mask”, “substrate” and “targetportion”, respectively.

In the present document, the terms “radiation” and “beam” are used toencompass all types of electromagnetic radiation, including ultravioletradiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm).

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in which:

FIG. 1 depicts a lithographic projection apparatus according to anembodiment of the invention;

FIG. 2 depicts a liquid supply system for supplying liquid to the areaaround the final element of the projection system according to anembodiment of the invention;

FIG. 3 depicts the arrangement of inlets and outlets of the liquidsupply system of FIG. 2 around the final element of the projectionsystem according to an embodiment of the invention;

FIG. 4 depicts a liquid supply system with bubble reduction meansaccording to an embodiment of the invention;

FIG. 5 depicts two possible arrangements of ultrasonic transducers in abubble detection means according to two embodiments of the invention;

FIG. 6 depicts an arrangement of ultrasonic transducers and standingwaves in a bubble removal means according to an embodiment of theinvention;

FIG. 7 depicts a degassing device according to an embodiment of theinvention;

FIG. 8 depicts a liquid pressurization device according to an embodimentof the invention;

FIG. 9 depicts an embodiment of the bubble removal means showing a pairof protected electrodes and associated electric field generator;

FIG. 10 illustrates several different embodiments of the presentinvention with a different liquid supply system to that illustrated inFIGS. 2 and 3;

FIGS. 11 a and 11 b depict an embodiment of the bubble removal meansarranged to selectively heat bubbles via a microwave radiation source;

FIG. 12 depicts an embodiment of the bubble removal means comprising aparticle input device and a particle removal device;

FIG. 13 depicts an embodiment of the bubble detection means showing thelight source and light detector and an example trajectory for a beam oflight scattered from its path within the liquid through the projectionlens to the light detector;

FIG. 14 depicts a larger scale view of the substrate region of thearrangement shown in FIG. 13, illustrating the introduction of lightfrom the light source into the region between the final element of theprojection lens and the substrate, according to a first embodiment ofthe light source;

FIG. 15 depicts the same view as FIG. 14 but shows the introduction oflight from the light source into the region between the final element ofthe projection lens and the substrate according to a second embodimentof the light source;

FIG. 16 depicts an embodiment of the bubble detection means comprising alight source, detector, light comparator, liquid quality monitor andalarm; and

FIG. 17 depicts an arrangement of ultrasonic transducers in the regionof the final element of the projection lens and the substrate accordingto an embodiment of the invention.

In the Figures, corresponding reference symbols indicate correspondingparts.

DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS Embodiment 1

FIG. 1 schematically depicts a lithographic projection apparatusaccording to a particular embodiment of the invention. The apparatuscomprises:

-   -   a radiation system Ex, IL, for supplying a projection beam PB of        radiation (e.g. DUV radiation), which in this particular case        also comprises a radiation source LA;    -   a first object table (mask table) MT provided with a mask holder        for holding a mask MA (e.g. a reticle), and connected to first        positioning means for accurately positioning the mask with        respect to item PL;    -   a second object table (substrate table) WT provided with a        substrate holder for holding a substrate W (e.g. a resist-coated        silicon wafer), and connected to second positioning means for        accurately positioning the substrate with respect to item PL;    -   a projection system (“lens”) PL (e.g. a refractive lens system)        for imaging an irradiated portion of the mask MA onto a target        portion C (e.g. comprising one or more dies) of the substrate W.

As here depicted, the apparatus is of a transmissive type (e.g. has atransmissive mask). However, in general, it may also be of a reflectivetype, for example (e.g. with a reflective mask). Alternatively, theapparatus may employ another kind of patterning means, such as aprogrammable mirror array of a type as referred to above.

The source LA (e.g. an excimer laser) produces a beam of radiation. Thisbeam is fed into an illumination system (illuminator) IL, eitherdirectly or after having traversed conditioning means, such as a beamexpander Ex, for example. The illuminator IL may comprise adjustingmeans AM for setting the outer and/or inner radial extent (commonlyreferred to as σ-outer and σ-inner, respectively) of the intensitydistribution in the beam. In addition, it will generally comprisevarious other components, such as an integrator IN and a condenser CO.In this way, the beam PB impinging on the mask MA has a desireduniformity and intensity distribution in its cross-section.

It should be noted with regard to FIG. 1 that the source LA may bewithin the housing of the lithographic projection apparatus (as is oftenthe case when the source LA is a mercury lamp, for example), but that itmay also be remote from the lithographic projection apparatus, theradiation beam which it produces being led into the apparatus (e.g. withthe aid of suitable directing mirrors); this latter scenario is oftenthe case when the source LA is an excimer laser. The current inventionand claims encompass both of these scenarios.

The beam PB subsequently intercepts the mask MA, which is held on a masktable MT. Having traversed the mask MA, the beam PB passes through thelens PL, which focuses the beam PB onto a target portion C of thesubstrate W. With the aid of the second positioning means (andinterferometric measuring means IF), the substrate table WT can be movedaccurately, e.g. so as to position different target portions C in thepath of the beam PB. Similarly, the first positioning means can be usedto accurately position the mask MA with respect to the path of the beamPB, e.g. after mechanical retrieval of the mask MA from a mask library,or during a scan. In general, movement of the object tables MT, WT willbe realized with the aid of a long-stroke module (course positioning)and a short-stroke module (fine positioning), which are not explicitlydepicted in FIG. 1. However, in the case of a wafer stepper (as opposedto a step-and-scan apparatus) the mask table MT may just be connected toa short stroke actuator, or may be fixed.

The depicted apparatus can be used in two different modes:

In step mode, the mask table MT is kept essentially stationary, and anentire mask image is projected in one go (i.e. a single “flash”) onto atarget portion C. The substrate table WT is then shifted in the x and/ory directions so that a different target portion C can be irradiated bythe beam PB;

In scan mode, essentially the same scenario applies, except that a giventarget portion C is not exposed in a single “flash”. Instead, the masktable MT is movable in a given direction (the so-called “scandirection”, e.g. the y direction) with a speed v, so that the projectionbeam PB is caused to scan over a mask image; concurrently, the substratetable WT is simultaneously moved in the same or opposite direction at aspeed V=Mv, in which M is the magnification of the lens PL (typically,M=¼ or ⅕). In this manner, a relatively large target portion C can beexposed, without having to compromise on resolution.

FIGS. 2 and 3 depict a liquid supply system according to an embodimentof the invention and have been described above. Other liquid supplysystems may be employed according to embodiments of the inventionincluding, without limitation, a bath of liquid and seal member asdescribed above.

FIG. 4 shows the liquid supply system 1 and the bubble reduction means 3a/3 b according to an embodiment of the invention. The bubble reductionmeans 3 a/3 b may be located underneath the projection lens 3 a, orexterior to the imaging axis 3 b. The liquid supply system 1 suppliesliquid to a reservoir 13 between the projection lens PL and the wafer W.The liquid is preferably chosen to have a refractive index substantiallygreater than one meaning that the wavelength of the projection beam isshorter in the liquid than in air or a vacuum, allowing smaller featuresto be resolved. It is well known that the resolution of a projectionsystem is determined, inter alia, by the wavelength of the projectionbeam and the numerical aperture of the system. The presence of theliquid may also be regarded as increasing the effective numericalaperture.

If the liquid has been exposed to the atmosphere, some atmospheric gasesmay be dissolved in the liquid. Disturbances of the fluid (in any way)may give rise to the formation of bubbles, which, depending on theliquid, gases and disturbances involved, may be very fine. Fine bubbles,down to around 10 nm in diameter, are very difficult to detect usingstandard methods but still interfere with the imaging performance of theexposure radiation, distorting the image and leading to printing defectson the wafer. Bubbles may also enter the reservoir 13 via out-gassingfrom elements within the lithographic apparatus such as thephotosensitive layer on the substrate W when it is exposed.

The reservoir is bounded at least in part by a seal member 17 positionedbelow and surrounding the final element of the projection lens PL. Theseal member 17 extends a little above the final element of theprojection lens PL and the liquid level rises above the bottom end ofthe final element of the projection lens PL. The seal member 17 has aninner periphery that at the upper end closely conforms to the step ofthe projection system or the final element thereof and may, e.g., beround. At the bottom, the inner periphery closely conforms to the shapeof the image field, e.g. rectangular but may be any shape.

Between the seal member 17 and the wafer W, the liquid can be confinedto the reservoir by a contact-less seal 16, such as a gas seal formed bygas, e.g. nitrogen, argon, helium or similar that do not readilydissolve into the liquid, provided under pressure to the gap between theseal member 17 and the substrate W. Between the seal member 17 and theprojection lens PL, the liquid is confined by sealing members 14,optionally to keep the liquid pressurized. Alternatively, the sealingmembers 14 may be omitted and the liquid confined by gravity.

The bubble reduction means 3 can comprise bubble removal means. FIG. 4shows an aspect of the bubble removal means, wherein the liquid is madeto flow continuously past the projection lens PL and substrate W. Thisaction is particularly effective for transporting away bubbles from gasoriginating within the reservoir 13, e.g. those arising due toout-gassing from the substrate W. Liquid is introduced to the reservoir13 through channels 23 formed at least partly in the seal member 17.These channels 23 may cooperate with channels for feeding thecontact-less seal 16, which may consist of inlet and outlet ports forgas and/or liquid. For example, liquid may be sucked from the region ofthe reservoir nearest the contact-less seal 16 by a gas outlet port andarranged to feed the continuous flow.

The bubble reduction means 3 can comprise bubble detection means 4. FIG.5 shows two arrangements of ultrasonic transducers 5 a/5 b in the bubbledetection means 4. The principle of detection used here is that theultrasonic wave amplitude will be attenuated due to Rayleigh scatteringfrom bubbles in the liquid. The ultrasonic attenuation is a function ofthe size distribution and the number density of bubbles (i.e. the numberper unit volume). In the left diagram, an ultrasonic transducer emits apulse that, after passing through the immersion liquid and reflectingfrom a boundary within the reservoir (whether reservoir 13 or some otherreservoir, for example exterior to the imaging axis), is received by thesame transducer 5 a. This arrangement of transducer 5 a is known as a“pulse-echo” arrangement. The pulse-echo arrangement is effectivebecause it only requires a single transducer 5 a and it is relativelyeasy to have a large propagation path between emission and detectionthus helping to maximize the sensitivity to bubbles. However, it ispossible that anomalous reflections occur causing loss of signal. Thesampling rate may also be limited by the fact that it is necessary towait for the return of a pulse before emitting a further pulse.Arranging the transducer 5 a so that it can emit and receiveconcurrently may obviate this problem. An alternative arrangement isshown on the right of FIG. 5, using two transducers 5 b each dedicatedto either emitting or receiving ultrasonic waves. Here it is possible toemit rapid trains of pulses and the arrangement does not suffer fromanomalous reflection effects since the wave pulses travel directlybetween the transducers 5 b.

The attenuation is measured as a function of frequency in order todetect bubbles that are much smaller than the wavelength of theultrasonic signals. This may be done using broadband transducers andexcitations. Measuring attenuation at only a single frequency restrictsdetection to bubbles with diameters of the same order of size as orlarger than the wavelength of the ultrasonic signals.

FIG. 6 shows a further aspect of the bubble removal means according toan embodiment of the invention, wherein two ultrasonic transducers 5 cpowered by a signal generator 9 and phase shifted relative to each otherby phase adjusting means 8 are arranged to produce a standing wavepattern 6 in the liquid between the faces of the transducers 5 c. FIG. 6shows a standing wave made up of interfering sine waves but the standingwaves may be of any periodic form (e.g. square-wave or saw-tooth). Theupper diagram represents the arrangement at a first instant and thelower diagram the same arrangement at a later instant. Bubbles presentin the liquid (e.g. 2) tend to become localized near the nodal regions 7of the standing wave 6. The phase adjusting means 8 act to shift thepositions of the nodes towards one or the other of the two ultrasonictransducers 5 c as shown by arrow 25. The trapped bubbles 2 move alongwith the moving nodes towards the transducer 5 c in question and aretherefore transported to an edge of a liquid reservoir. In FIG. 6, thismovement is to the left as indicated by the arrow 26 and thedisplacement of the sample trapped bubble 2 indicated by the displacedvertical broken lines that pass through the center of the trapped bubble2 at the two consecutive times. Once a certain concentration of bubbleshas accumulated near one transducer 5 c, the liquid in this region maybe isolated and removed from the reservoir, carrying the bubbles withit.

The bubble removal means may work using ultrasonic waves as described inEuropean Patent Application No. 03253694.8 hereby incorporated in itsentirety by reference, or on similar principles using higher frequencywaves known as megasonic waves (about 1 MHz) which avoid some of thedisadvantages of conventional ultrasonic waves (which can lead tocavitation and bubble collision with walls resulting in small particlesbreaking off the walls and contaminating the liquid). As an alternative,the ultrasonic energy may be controlled, even with lower frequencyultrasound, to reduce the likelihood or extent of bubble cavitation.Additionally, ultrasound may be used to cause coalescence of smallerbubbles into larger bubbles which rise more quickly and may be moreeasily removed. Other bubble reduction means are also possible, forexample those described in the above mentioned European PatentApplication as well as the use of membranes perhaps in combination witha vacuum or by purging the liquid with a low solubility gas, such ashelium. Membranes are already used for removal of gases from liquids infields such as microelectronics, pharmaceutical and power applications.The liquid is pumped through a bundle of semi porous membrane tubing.The pores of the membrane are sized and the material chosen so that theliquid cannot pass through them but the gases to be removed can. Thusthe liquid is degassed. The process can be accelerated by applying tothe outside of the tubing a low pressure. Liqui-Cel (™) MembraneContractors available from Membrana-Charlotte, a division of CelgardInc. of Charlotte, N.C., USA are suitable for this purpose.

Purging with a low solubility gas is a known technique applied in highperformance chromatography to prevent air bubble trapping in areciprocating pump head. When the low solubility gas is purged throughthe liquid, it drives out other gases, such as carbon dioxide andoxygen.

FIG. 7 shows the degassing device 10 of the bubble removal meansaccording to an embodiment of the invention. The degassing device 10comprises an isolation chamber 11, which contains the liquid to bedegassed. The degassing device 10 may further comprise a pump 12arranged to extract gases from the isolation chamber 11 and, eventually,to achieve a low pressure state therein. The minimum pressure ispreferably chosen to be greater than the saturated vapor pressure of theliquid being used so as to prevent boiling, e.g. around 23 mbar forwater at room temperature. Once under reduced pressure, gases dissolvedin the liquid will leave solution and be pumped away by the pump 12.Raising the temperature of the liquid can assist this process. Forexample, working between 40 and 50° C. typically increases the degassingspeed by about a factor of ten. When the degassing process is complete,i.e. when no further dissolved gas can be extracted from the liquid, theisolation chamber 11 may be isolated by closing doors 15 located abovethe liquid. The liquid should remain isolated from the atmosphere untilit is transferred into the reservoir 13 for use. The liquid may be kepteither under vacuum or under a gas that will not easily dissolve intothe liquid, such as nitrogen, argon or helium.

FIG. 8 shows a liquid pressurization device 22 that acts to pressurizethe reservoir liquid above atmospheric pressure according to anembodiment of the invention. High pressure has the effect of minimizingthe size of bubbles and encouraging bubbles to dissolve into the liquid.The apparatus shown in FIG. 8 consists of a piston 19 and a bore 21.Pushing the piston into the bore pressurizes the liquid. At its lowerend, a valve 18 is provided to allow transfer of the liquid, for exampleinto the liquid supply system 1. For monitoring purposes, a pressuregauge 20 is provided which may include a safety blow-off valve.

The bubble reduction means 3 may comprise elements both within thereservoir 13, as shown in FIG. 4, and outside the reservoir 13—see 3 aand 3 b respectively in FIG. 4. An advantage of having elements outsidethe exposure space 13 is that engineering considerations, such as theamount of space available or the allowable levels of vibrations and heatdissipation, are significantly relaxed. This fact not only makes itcheaper to design processing elements but also opens the possibility forbulk processing. Such bulk processing may allow a single station toprepare liquid for use in a number of lithographic apparatuses or toprovide a large quantity of conditioned liquid for use in a system wherethere is a continual throughput of liquid, or in a system where theliquid is changed on a frequent basis.

Bubble reduction means 3 located within the reservoir 13 areparticularly effective for dealing with bubbles that unavoidablyoriginate within the reservoir 13, such as from out-gassing.

The composition of the liquid may be chosen to have a lower surfacetension than water. This reduces the tendency of bubbles to stick to thesubstrate (particularly acute for small bubbles) where they may beparticularly damaging to the image and where they tend to be resistantto removal measures. This may be achieved by choosing a pure liquid witha lower surface tension or by adding a component to the liquid thatreduces its surface tension, such as a surfactant.

Bubbles attached to the surface of the substrate W are particularlydamaging because they are near the focus of the projection apparatus.The image is thus liable to be seriously distorted due to diffraction.An embodiment of the present invention provides a means for removingsuch bubbles and, more generally, bubbles attached to any interfaceswithin the immersion liquid. FIG. 9 illustrates one such embodiment, inthis case directed towards removing bubbles from the substrate W. Here,two electrodes 27 a and 27 b are arranged in the region between thefinal element of the projection system PL and the substrate W and areeach connected to terminals of an electrical power source 28.Alternatively, parts of the existing apparatus may be utilized aselectrodes. For example, the substrate W may form one electrode inpartnership with a second such as 27 a. When energized, this arrangementproduces a uniform electric field substantially parallel to the axis ofthe projection lens PL which extends to the region of liquid in closeproximity to the target interface. Bubbles have a dielectric constantdifferent from that of the surrounding liquid, which causes electricfield lines to be distorted in the region around the bubble. Whenbubbles are close to an interface such as the substrate (W), the fieldlines may be distorted in such a way that the bubble experiences aforce, which may be directed away from the surface in question and causethe bubble to deform and eventually break free from the surface andenter the bulk of the liquid. In the context of FIG. 9, the magnitude ofthe electric field may be arranged to overcome the pressure exerted onthe bubble due to the liquid located above it and other opposing forcesoriginating from factors such as surface tension. In a preferredembodiment the potential difference between the electrodes 27 a and 27 bis 100 volts DC. However, alternating voltage sources or a combinationof alternating and direct voltage sources may be used. The criticalparameter is the electric field strength, which depends on the magnitudeof the potential difference and the separation between the electrodes.Furthermore, non-uniform and differently oriented fields may also beeffective. This method will be applicable even when the surface of thesubstrate W is hydrophobic and there is a large energy barrierassociated with deforming the bubble and disconnecting it from thesurface. This means that it is no longer necessary specially to treatthe surface of the substrate W such as by coating it with a hydrophiliccoating.

A number of design considerations need to be taken into account. Theconductivity of the liquid needs to be carefully controlled. Inparticular, it should not be too high, because this will make itdifficult to create the electric field. Water with a resistivity ofroughly 0.8 to 18.2 MOhm*cm may be used for example. Also, theelectrodes 27 a and 27 b should preferably be protected from breakdownby isolating material 29 to prevent electrolysis and subsequent materialbreakdown. The conductivity and/or dielectric permittivity of theelectrodes themselves should be high in comparison to the immersionliquid. One consequence of this will be to ensure that there is noappreciable fall in potential within the conductor material, which mayhelp produce a uniform field between the electrodes.

It has been found that electrical forces may also cause adhesion betweenbubbles and solid particles dispersed in liquid. Bubbles in a liquidhave, on their surface, an electrokinetic (or zeta) potential whichresults in a potential difference between the surface of the bubble andthe fully disassociated ionic concentration in the body of the liquid.This also applies to small particles.

According to an embodiment of the present invention, a power source orvoltage supply V (or charge, voltage, electrical field or potentialdifference generator or supply) may be used to apply an electricalpotential to one or more objects of the immersion apparatus. Theprinciple of operation is that if repulsion is required a potentialdifference between the fully disassociated ionic concentration of theliquid and the object is generated, which is of the same polarity as thepotential difference between the fully disassociated ionic concentrationin the body of the liquid and the surface of the bubble. If attractionbetween the object and the bubble is required the potential differencesshould have the same polarity. In this way forces can be generated onthe bubbles towards or away from the objects (electrodes) which are incontact with the immersion liquid.

In FIG. 10 several different objects have a potential or charge appliedto them. This embodiment will work with only one such object and alsowith any combination of objects and indeed other objects to those notillustrated could be also or alternatively be used.

In pure water, which is the most promising candidate for use as animmersion liquid at 193 nm projection beam wavelength, it has been foundthat the surface potential of μm bubbles is about −50 mV. This potentialwill vary with bubble size and also with type of immersion liquid.However, the same principles as described here can be used for otherimmersion liquids and bubble sizes and the invention is fully applicableto those. Additives may be added to the immersion liquid to change theeffect of the potential. CaCl₂ or NaCl are suitable candidate additionsfor this purpose.

In FIG. 10 six different objects are illustrated to which a potential orvoltage or charge could be applied. Preferably the objects are incontact with the immersion liquid. Though in principle this is notnecessary. One of these is the substrate W which is preferably chargedto the same polarity of electrical potential as the electrical potentialof the surface of the bubbles. In this way the bubbles have a force onthem directly away from the substrate W so that their effect on theprojected image is minimized. In combination with a negative potentialon the substrate W, or by itself, the final element of the projectionsystem or an object 50 close to the final element of the projectionsystem PL can be charged to a potential opposite in polarity to thepotential of the surface of the bubbles. This will have the effect ofattracting the bubbles towards the final element of the projectionsystem and thereby away from the substrate. The shape of the object 50(electrode) close to the final element of a projection system PL couldbe any shape. It could be plate like or could be annular so that theprojection beam PB passes through the centre of electrode 50.

Alternatively, the objects to be charged or have a voltage applied tothem could be attached to a surface of the seal member 17. In FIG. 10,these objects are attached to the inner surface of the seal member 17.As illustrated, two electrodes 52, 54 are present each on opposite sidesof the barrier member and charged to opposite potentials. In this waythe bubbles could be drawn to one or other of the objects, perhaps inthe direction of an immersion liquid outlet. Alternatively, one objector more objects may be provided around the inner side of the seal member17 (in contact with the immersion liquid) which is/are charged to apotential with a polarity different to the polarity of the potential ofthe surface of the bubbles. In this way bubbles in the immersion liquidin the space 36 between the final element of the projection system PLand the substrate W will be drawn away from the optical axis of theapparatus thereby leaving the path of the projection beam PB to thesubstrate W substantially unhindered by bubbles.

Another place to use this embodiment is upstream of the space 36 betweenthe final element of the projection system PL and the substrate W in theliquid supply system. In this case, as the immersion liquid passes alongconduits 56 and through a housing 58, oppositely charged and opposingplates 62, 64 produce a force on the bubbles which is effective to movethe bubbles, when the immersion liquid is in the space 36, further awayfrom the substrate W than they would be without the application of theelectrical field upstream of the space 36. The immersion liquid with ahigh concentration of bubbles i.e. near to the electrode 64, could evenbe removed and not supplied to the space 36. The removed liquid could besubjected to a bubble removal process before being recycled in theliquid supply system.

In all of the above examples, the higher the voltage applied by thevoltage generator V the greater the force on the bubbles. The potentialon the objects should not be so high as to cause disassociation of theimmersion liquid but should be high enough to provide a force on thebubbles such that the invention is effective. For an immersion liquidcomprised mainly of water, typical potential differences applied to theobjects according to this embodiment are 5 mV to 5V, preferably 10 mV to500 mV. An electrical field of 5 mV/mm to 500 mV/mm due to theapplication of the potential is preferred.

FIG. 11 illustrates an embodiment of the bubble removal means thatbenefits from a significantly enhanced bubble removal rate without undueinfluence on the immersion liquid. The improved removal rate is achievedby increasing the size of the bubbles in the immersion liquid byheating. The increased bubble size renders them more responsive to mostmethods of bubble removal. This is achieved without adverse heatingeffects in the immersion liquid, or surrounding temperature sensitivecomponents, through the use of a microwave radiation source 30,producing radiation that couples only to the gas within the bubblesthemselves and not to the immersion liquid itself. FIG. 11 a, whichshows a schematic magnified view of the immersion liquid, illustratinghow the process operates. Microwave photons 32 are absorbed by anexample bubble 31 a at temperature T1, which is then heated to become alarger bubble 31 b at temperature T2. Once the temperature of the bubblehas been elevated above that of the surrounding immersion liquid, somerise in the temperature of the immersion liquid will inevitably occur inthe immediate vicinity of each bubble. However, the combined heatcapacity of the bubbles and thermal conductivity of the immersion liquidare likely to be small enough that heating of the immersion liquid canbe kept within acceptable limits. In general, the frequency componentsof the microwave radiation are chosen to correspond with resonantfrequencies or excitation modes of species present in the bubbles. Formany cases of interest, a large fraction of the gas forming the bubbleswill be nitrogen and oxygen, in which case the resonant modes of thesemolecules will dictate the microwave frequencies to use.

FIG. 12 illustrates an alternative embodiment of the bubble removalmeans. Here, a particle input device 33 introduces into the immersionliquid particles that act to attract bubbles to their surface. Theparticles may be mixed with the immersion liquid either by naturaldispersion or deliberate agitation. The particles may be left in theimmersion liquid for a period determined according to the concentrationof bubbles. For example, if the bubble concentration is very high, theparticles will become saturated quickly and will need to be refreshedafter a relatively short time. If, on the other hand, the bubbleconcentration is low, the particles may remain active for a much longertime. Once the activity of the particles, or alternatively the bubbleconcentration, has fallen below a certain threshold level, the particlesmay be removed from the liquid by a particle removal device 34, whichmay comprise for example a particle filter. According to the embodimentof FIG. 11, the particle input device 33 and particle removal device 34are coupled to the channels 23 for circulating the immersion liquidthrough the region 36 through a circuit indicated by the arrows 37 and38. The circuit in question may be closed, as indicated by arrows 38, orinvolve input and output to a mains, or other, water supply as indicatedby arrows 37. The used particles may be treated in a particle recyclingdevice 35 to remove the gas bubbles from the particles. This de-gassingprocess may be achieved, for example, by pumping on a solutioncontaining the particles or by pumping directly on the particlesthemselves. The clean particles may then be reintroduced to theimmersion liquid via the particle input device 33 where they will againact effectively to trap bubbles.

Preferably the particles are arranged to have surface characteristicsthat encourage bubbles to attach to the surface, for example in order tolower their surface energy. In addition, it may also be preferable toarrange the particles to have as large a surface area as possible. Thismay be achieved by using porous particles such that bubbles may attachon surfaces within the interior of the particles. Generally thisparameter may be varied by controlling the size and number distribution,and porosity of the particles. A balance may need to be achieved in poresize because although finer pores may provide the greatest additionalsurface area, they will exclude bubbles that are of a similar order ofsize or that are large in comparison with the pores (the pores may alsobe blocked by such bubbles). Many different particle compositions can beused, for example silica, zeolites, alumina, activated carbon, or acarbon molecular sieve. Certain polymer compositions may also be used.The particle size is a less critical factor (compared with the surfacearea) and a typical size range may be 5 to 1000 μm diameter.

In FIG. 12, the particle input device 33 and the particle removal device34 are both located outside of the region 36. However, these componentsmay also be arranged to add and remove particles directly within thisregion.

An alternative method for bringing the particles into the liquid is theoccasional use of ultrasonic agitation in combination with non-degassedliquid. Due to cavitation of the bubbles, particles will be releasedfrom solid surfaces exposed to the liquid.

FIG. 13 shows a schematic representation of a section of thelithographic projection apparatus between the mask MA and the substrateW. This diagram shows several possible embodiments of the inventionwherein either the bubble detection means or a detection system isarranged to propagate light between a light source 39 and a detector 40.The presence of bubbles (in the case of the bubble detection system) orparticles (in the case of the detection system) is established via anincrease or decrease in the intensity of light reaching the detector 40,caused by light scattering from bubbles or particles within the liquid.FIG. 13 shows one possible arrangement, with the light source 39arranged via an optical fiber 41 to direct light rays into the immersionliquid. Light propagates through the liquid and, if bubbles or particlesare present, may scatter from them. An example path for a scattered rayis shown by the arrows 42, showing propagation through the projectionlens system to the detector 40. Preferably, a wavelength is chosen suchthat the photo resist is insensitive to the light. FIGS. 14 and 15 showmagnified views of the substrate region showing how the light is fedinto the immersion liquid. In FIG. 14, the optical fiber 41 is fedthrough the seal member 17 and makes its way into the region 36 eitherdirectly or after a number of reflections. FIG. 15 shows an alternativearrangement whereby light is introduced between the substrate W and theseal member 17. In FIGS. 14 and 15, light is shown (by arrows 43 a and43 b) entering from a single direction and traversing the region 36horizontally. However, light may be fed into the liquid from anydirection and take various paths including paths comprising one or morereflections off the final element of the projection system PL and/or thesubstrate W. According to the embodiment illustrated in FIGS. 13 to 15,the signal strength detected at the light detector will increase as theconcentration of bubbles or particles in the liquid increases due to theoverall increase in scattering. However, the light source 39 anddetector 40 may be arranged so that increased scattering leads to adecrease in the signal strength arriving at the detector 40. As afurther variation, the optical fiber 41 may connect to both anillumination source and detector, the presence of bubbles or particlesbeing detected by a change in the amount of light being reflected backinto the optical fiber 41.

The arrangement illustrated in FIGS. 13 to 15, which in general may bedescribed as a light scatterometer, has the advantage of allowingcontinuous and non-disruptive monitoring of the concentration of bubblesor particles in the immersion liquid. FIG. 16 illustrates schematicallyhow the arrangement may be realized, with the light source 39 anddetector 40 interacting with a light comparator 44. The light comparator44 compares the light emitted by the light source 39 and the signallevel arriving at the detector 40, and, depending on the arrangement ofsource and detector, determines information about the population ofbubbles or particles present in the immersion liquid.

The light comparator 44 may interact with a liquid quality monitor 45,which may be realized by a suitably programmed computer. The liquidquality monitor 45 may be arranged to ensure that the liquid is alwaysat a suitable level of cleanliness to ensure that the quality of theimage being written to the substrate W does not fall below a minimumthreshold level. The liquid quality monitor 45 may take into account, inaddition to the concentration of bubbles or particles, other factorssuch as the chemical composition of the liquid. The liquid qualitymonitor 45 may in turn be coupled to an alarm system 46 that causes thesystem to be shut down from an active state to a suspended state, orother appropriate action to be taken, when the state of the immersionliquid falls outside predefined parameters. This early reaction toproblems in the liquid allows corrective action to be taken promptly,and also minimizes the loss of materials and time associated withsubstandard exposures caused by low quality immersion liquid.

The imaging performance of the lithography system may also be affectednegatively (causing stray light, for example) by contamination on thebottom part of the lens PL. Such contamination may include, for example,the formation of salts arising primarily from the resist chemicals oroxides such as SiO₂. The contamination may be reduced by mechanical orchemical cleaning, but such procedures involve expensive stoppages andservice man hours, are not always completely effective and risk damageto the lens. According to certain embodiments of the present inventiondescribed above, one or more ultrasonic transducers are provided todetect or remove bubbles from the immersion liquid. These devices mayalso be oriented and configured to remove contamination from the finalelement of the projection lens PL and the substrate or wafer chuck W.FIG. 17 shows one possible arrangement, wherein ultrasonic transducers47 are located on the seal member 17 and may couple directly to theliquid between the final element of the projection lens PL and thesubstrate W. To minimize the risk of altering the position of the lensitself during cleaning, the transducers 47 may be mechanically isolatedfrom, or at least in damped connection with, the seal member 17. Forexample, the transducers 47 may be located nearby, rather than on, theseal member 17. Alternatively, the device connection to the lens PL maybe mechanically released when the high frequency is generated. In thecontext of lens or wafer chuck cleaning, a wide variety of highfrequency generators may be used that produce ultrasonic waves resonantwith the immersion liquid. In practice, the ultrasonic lens and waferchuck cleaning action may be implemented automatically and be arrangedto cycle on and off according to the rate of contamination.

Whilst specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The description is not intended to limit theinvention.

1. A lithographic projection apparatus comprising: a radiation systemfor providing a projection beam of radiation; a support structure forsupporting patterning means, the patterning means serving to pattern theprojection beam according to a desired pattern; a substrate table forholding a substrate; a projection system for projecting the patternedbeam onto a target portion of the substrate; a liquid supply system forat least partly filling a space between the final element of saidprojection system and said substrate with liquid, wherein said liquidsupply system comprises bubble reduction means.
 2. A lithographicprojection apparatus according to claim 1, wherein said bubble reductionmeans comprise bubble detection means.
 3. A lithographic projectionapparatus according to claim 2, wherein said bubble detection meanscomprise at least one ultrasonic transducer, the attenuation ofultrasonic waves in said liquid being measured by said transducer so asto obtain information about bubbles present in said liquid.
 4. Alithographic projection apparatus according to claim 3 wherein saidultrasonic transducer measures ultrasonic attenuation as a function offrequency.
 5. A lithographic projection apparatus according to claim 1,wherein said bubble reduction means comprise bubble removal means.
 6. Alithographic projection apparatus according to claim 5, wherein saidbubble removal means comprise a degassing device, said degassing devicecomprising an isolation chamber, wherein a space above liquid in saidisolation chamber is maintained at a pressure below atmospheric pressureencouraging previously dissolved gases to come out of solution and bepumped away.
 7. A lithographic projection apparatus according to claim5, wherein said bubble removal means provides a continuous flow ofliquid over the final element of said projection system and saidsubstrate to transport bubbles in said liquid out of said space betweenthe final element of said projection system and said substrate.
 8. Alithographic projection apparatus according to claim 1, wherein saidbubble reduction means comprise a liquid pressurization device topressurize said liquid above atmospheric pressure to minimize the sizeof bubbles and encourage bubble-forming gases to dissolve into saidliquid.
 9. A lithographic projection apparatus according to claim 1,wherein the composition of said liquid is chosen to have a lower surfacetension than water.
 10. A lithographic projection apparatus according toclaim 1, wherein said bubble reduction means treat said liquid before itis supplied to said space between the final element of said projectionsystem and said substrate.
 11. A lithographic projection apparatusaccording to claim 10, wherein the treated liquid is kept in a sealedcontainer, excess space in said sealed container being filled with oneor more of the following: nitrogen gas, argon gas, helium gas or avacuum.
 12. A lithographic projection apparatus according to claim 3,wherein an ultrasonic transducer is arranged in a pulse-echoconfiguration, said transducer acting both to transmit ultrasonic wavesand, after reflection, to receive ultrasonic waves that have beenattenuated during propagation along a path through said liquid.
 13. Alithographic projection apparatus according to claim 3, wherein saidbubble detection means comprise two spatially separated ultrasonictransducers, the first arranged to transmit ultrasonic waves, and thesecond to receive ultrasonic waves that have been attenuated duringpropagation along a path through said liquid between the twotransducers.
 14. A lithographic projection apparatus according to claim5, wherein said bubble removal means includes two spatially separatedultrasonic transducers, arranged to produce ultrasonic standing-wavepatterns within said liquid which trap bubbles within the nodal regions,said bubble removal means being arranged to displace said bubblesthrough the use of phase-adjusting means linked with said transducers,said phase-adjusting means causing spatial shift of nodal regions andbubbles trapped therein.
 15. A lithographic projection apparatusaccording to claim 5, wherein said bubble removal means comprises anelectric field generator for applying an electric field to said liquid,said electric field being capable of dislodging bubbles attached to saidsubstrate.
 16. A lithographic projection apparatus according to claim 5,wherein said bubble removal means comprises a selective heater forselectively controlling the temperature and therefore size of bubbles ofa particular composition.
 17. A lithographic projection apparatusaccording to claim 16, wherein said selective heater comprises amicrowave source.
 18. A lithographic projection apparatus according toclaim 5, wherein said bubble removal means comprises a particle inputdevice for introducing particles into said liquid, and a particleremoval device for removing said particles from said liquid.
 19. Alithographic projection apparatus according to claim 18, wherein saidparticles comprise a surface with characteristics that encourage bubblesto attach thereto.
 20. A lithographic projection apparatus according toclaim 2, wherein said bubble detection means comprises a light source, alight detector and a light comparator, said light source and said lightdetector being arranged so that light emitted by said source propagatesbetween said source and said detector through a portion of said liquid,said comparator being arranged to detect changes in the proportion ofsaid emitted light that arrives at said detector after propagationthrough a portion of said liquid.
 21. A lithographic projectionapparatus comprising: a radiation system for providing a projection beamof radiation; a support structure for supporting patterning means, thepatterning means serving to pattern the projection beam according to adesired pattern; a substrate table for holding a substrate; a projectionsystem for projecting the patterned beam onto a target portion of thesubstrate; a liquid supply system for at least partly filling a spacebetween the final element of said projection system and said substratewith liquid; and a detection system for detecting impurities in saidliquid, including a light source, a light detector and a lightcomparator, said light source and said light detector being arranged sothat light emitted by said source propagates between said source andsaid detector through a portion of said liquid, said comparator beingarranged to detect changes in the proportion of said emitted light thatarrives at said detector after propagation through a portion of saidliquid.
 22. A lithographic projection apparatus according to claim 21,wherein said detection system is arranged to detect particles in saidliquid between said final element of the projection system and saidsubstrate.
 23. A device manufacturing method comprising: providing asubstrate that is at least partially covered by a layer ofradiation-sensitive material; providing a projection beam of radiationusing a radiation system; using patterning means to endow the projectionbeam with a pattern in its cross-section; projecting the patterned beamof radiation onto a target portion of the layer of radiation-sensitivematerial; providing a liquid supply system for at least partly filling aspace between the final element of said projection system and saidsubstrate with liquid; and reducing bubbles in said liquid supplysystem.
 24. A lithographic projection apparatus comprising: a radiationsystem for providing a projection beam of radiation; a support structurefor supporting patterning means, the patterning means serving to patternthe projection beam according to a desired pattern; a substrate tablefor holding a substrate; a projection system for projecting thepatterned beam onto a target portion of the substrate; a liquid supplysystem for at least partly filling a space between the final element ofsaid projection system and said substrate with liquid; and a liquidquality monitor capable of switching the operational state of theprojection apparatus between an active state and a suspended state, saidactive state being selected when the liquid quality is determined to beabove a predefined threshold and said suspended state being selectedwhen the liquid quality is determined to be below a predefined thresholdstate.